1. Field of the Invention
The invention relates to a process for producing a sulfide-based solid electrolyte useful as a solid electrolyte for, for example, all-solid-state lithium batteries, and more particularly, to a process for producing a sulfide-based solid electrolyte having superior production efficiency by eliminating the need to remove adhered substances formed on the inner surface of the milling pot during mechanical milling.
2. Description of the Related Art
Accompanying the rapid proliferation of information-related devices and communication devices such as personal computers, video cameras and cell phones in recent years, increasing emphasis has been placed on the development of batteries for use as power supplies of these devices. In addition, in the automobile industry as well, development is proceeding on high-output, high-capacity batteries for use in electric cars and hybrid vehicles. At present, among the various types of batteries available, lithium batteries are attracting attention from the viewpoint of their high energy density.
Since lithium batteries that are currently commercially available use organic electrolytes that use a flammable organic solvent as a solvent, the attachment of a safety device for inhibiting temperature rises during a short-circuit as well as improvements are in terms of structure and materials to prevent short-circuits are required.
In contrast, since all-solid-state lithium batteries, which replace the liquid electrolyte with a solid electrolyte to make the battery entirely solid state, do not use a flammable organic solvent therein, the safety device can be simplified thereby leading to reduced production costs and superior productivity. In addition, sulfide-based solid electrolytes are conventionally used for the solid electrolyte used in all-solid-state lithium batteries. Moreover, examples of processes used to produce sulfide-based solid electrolytes include mechanical milling and rapid melt cooling.
Japanese Patent Application Publication No. 11-134937 (JP-A-11-134937), for example, discloses a process for producing ion-conducting sulfide glass by vitrifying a raw material of ion-conducting sulfide glass by mechanical milling. Moreover, Japanese Patent Application Publication No. 2004-265685 (JP-A-2004-265685) and Japanese Patent Application Publication No. 2003-208919 (JP-A-2003-208919) disclose a process for producing lithium ion-conducting sulfide glass using mechanical milling. On the other hand, a process for producing a lithium ion-conducting solid electrolyte using rapid melt cooling is described in Japanese Patent Application Publication No. 9-283156 (JP-A-9-283156).
In general, mechanical milling offers the advantage of allowing a desired sulfide glass to be more easily obtained in comparison with rapid melt cooling since it enables treatment to be carried out at normal temperatures. However, raw material compositions of sulfide-based glass are comparatively soft due to the presence of sulfur (S) contained therein, and there was also the problem of adhered substances, including unreacted raw material composition, being formed on the inner surface of the milling pot during mechanical milling. Consequently, it was necessary to periodically remove adhered substances formed on the inner surface of the pot, thereby resulting in the problem of poor production efficiency.